[HBR] Another HBR Project -- Chapter 11

[hbr@mailman.qth.net]

Cleaning up details ...

A few hours listening to a new receiver always reveals things you'd 
rather not know.   After months of high noise levels on all bands, we 
had a quiet day on 40 recently and strong signals suddenly sounded 
raspy and harsh on the 1MHBR.   It didn't take a lot of thought -- 
acting on only two stages, the AGC (half 12AX7 operating as a plate 
detector, cathode run at -150V) didn't respond fast enough to prevent 
distortion on voice peaks, especially at the start of a sentence.   The 
12AX7 just couldn't charge the relatively large cap I used to get a 
long time constant, quickly enough.   The answer was a two-time-
constant system -- adding an 0.47 meg/0.1 mfd combination in 
series with the existing 4.7meg/0.5 mfd.   The fast tc takes care of 
sudden peaks; the long one keeps the gain down between words so 
you don't have to listen to background noise on strong signals.

The problem didn't show up during high noise levels because the 
noise kept the RF gain down enough to prevent overloading.

Of course the more complicated AGC circuit added a 'wrinkle' on the 
STANDBY switch wiring, but I finally got that done.   

The heat sink mounting of the front end tubes (on an L-shaped plate 
spaced 1/4" above the chassis) reduced the final coil temperature by 
3 or 4 degrees.   With the warm-up of the temp compensating cap at 
the oscillator socket slowed down by shortening the ground end lead 
from 1/2" to 1/4", the warm-up drift drops to 'almost okay' -- there's 
no overcompensation at the start and by the time I find a long-playing 
round table to listen to, retuning is required only every 15 minutes or 
so.   The next step is to move the compensating cap on the coil 
socket so it gets warmer as the set warms up -- I may be able to 
tuck it closer to the socket or the chassis.   

I'm pleased by the drift situation.   Any vacuum tube receiver that has 
oscillator parts spread over several inches of open chassis is going 
to have drift 'issues' but those of the 1MHBR are proving to be 
manageable, at least on the lowest bands.  The location of the coils 
in the panel, definitely helps -- they now warm up only about 15 
degrees above room temp and a substantial part of the heat is 
coming from the screws that hold the coil set to the front panel.  My 
hope is that the drift can be reduced to nil on 80 and 40 without 
compensation inside the coil cans.  (Maybe even 20, since the 
oscillator runs at about 7 Mcs.)   Then if necessary, the higher bands 
can be made 'okay' with small caps inside the cans.   I'm not much 
interested in 10, but maybe I'll wind the coils just to see how it goes 
up there, with the VFO working at 14 Mcs!

A receiver of this type can't be driftless, though.   If you can get it 
perfect when warming up from dead cold, then you'll always have 
noticible drift when you turn it off and then back on after an hour.   
The various parts cool down by different amounts in the first hour, so 
they're not starting from the same point when the set is turned on 
again.   The answer in that case is to use the 'STANDBY' switch for 
short intervals.

That's the big payoff in unit oscillators as in the Collins PTOs and 
Yaesu's FT-101 and other well-designed sets of that vintage.   
Everything that matters is shut up in one box with no heat sources 
inside so it all warms up and cools down at the same rate.   Once 
the unit is compensated, it's stable no matter what the ON-OFF 
pattern.

While making the AGC changes, I remembered one issue that could 
be a reason for IF instability or unwanted BFO pickup in these 
receivers.   We all grew up bypassing stuff with 0.01 mfd caps but 
when a stage operates at 85 kcs (or 100, as in the original HBR-
series) 0.01 isn't a bypass -- the reactance at 100 kcs is nearly 200 
ohms.   In a cathode or plate circuit, stuff is going to get past those 
caps.  The HBR 16 used 0.02 mfd in those locations; the -13 used 
0.05 and I've been using 0.1; for low voltages I sometimes use 0.47.   
It's pretty much impossible in these LF stages to use too large a cap.

I'm grinding my teeth now over the 20 meter coil set.   The worst of 
the coil trial-and-error is done but the sensitivity is zilch, even with 
toroids for the antenna and RF coils.  The problem is most likely the 
low 1st oscillator injection.  I'm betting that the HBR-series got 
around this with their use of a tiny coupling cap to the mixer so the 
2nd harmonic is a greater fraction of the total.  It's 2 mmf in the -11, 
13; a small 'gimmick' (typically a fraction of a mmf) in the -16 and in 
all cases goes to a grid that is tuned to the signal frequency and 
thus encourages the desired harmonic.   I put the signal and 1st 
oscillator into different grids ... this is going to take some thought.

Walt
KJ4KV